1. Field of the Invention
The invention relates to a system comprising a first device arranged to communicate with a second device. The first device can be, for example, a USB host. The second device can be, for example, a USB device, which communicates with the USB host via a USB bus using the USB protocol.
2. Background Art
The system comprises a USB host, which is connected to various USB devices via a USB bus. The USB host communicates with the USB devices with the USB protocol. The USB protocol allows connecting several USB devices on the same USB bus using a system of time-sharing based on addressed devices. The USB protocol is organized as a master/slave architecture, the USB host is thus responsible of the time-sharing management.
The USB host may comprise various applications. One or several services may be needed to run an application. An application uses one or several drivers to access and use the associated services. The drivers may be on the USB host.
The USB device may comprise various services, in particular those, which are needed by the applications of the USB host. A service may be offered, for example, at the device level (standard USB device), or at the interface level (composite USB device).
The USB device is organized into several levels a device level, a configuration level, an interface level and an endpoint level. Each level is represented by different USB descriptors:                A device descriptor describing the overall device. The device descriptor may be associated to one or more configuration descriptors.        A configuration descriptor describing the electrical characteristics of the USB device, or of a part of the USB device. The configuration descriptor may be associated to one or more interface descriptors.        An interface descriptor describing a particular service of the USB device. An interface may contain one or more alternate settings. The interface descriptor may be associated to zero or more endpoint descriptors.        An endpoint descriptor describing a communication channel used by the service defined by the interface descriptor.        
In a plugging step, the USB device is plugged onto a USB port of the USB host.
In an enumeration step, all the USB descriptors are then retrieved from the USB device to the host device. The enumeration step is triggered off with the modification of the voltage level on the line D+ or D− (depending on the USB device speed) due to a pull-up resistor present in the USB device on one of the lines.
In a loading step, the USB Host then uses the descriptors to load all the drivers of the USB device. The number of drivers loaded depends on the number of different services present in the USB device. For example, if the USB device is at the same time a scanner and a printer the USB device will have to present two interfaces during the enumeration step. In that case, two drivers, one associated to the scanner interface, and the other one associated to the printer interface, will be loaded. A main driver associated to the device itself could also be loaded.
US 2001/0027500 discloses a data transmission system comprising a host, a controller connected to the host via a plug and play compatible bus, and a plurality of functions provided by the connected controller, that exceed in number the maximum end points that the controller can support, wherein the controller selects a predetermined number of functions from among the plurality of functions and enumerates, as end points, the functions that match in number the maximum end points, and wherein thereafter, the controller replaces with a different function one of the functions that constitutes the end-points, and re-enumerates the endpoints.